This invention relates to regenerated starch in fiber form. More particularly, the invention relates to water insensitive starch fibers prepared from modified or unmodified starches containing from about 55 to 100% by weight amylopectin, and to a process of making the same.
Starch is a polymer comprising a plurality of anhydroglucose units arranged in one of two structural forms: as a linear chain polymer called amylose or as a highly branched polymer called amylopectin. The properties of these two forms of starch differ and much of the difference may be traced to the affinity of the hydroxyl groups in one particular structural molecule for those in another. Thus, in linear chain polymers such as amylose, the straight chains can orient in parallel alignment so that a large number of the hydroxyl groups along one chain are in close proximity to those on adjacent chains. When this happens, the hydroxyl groups form associations through hydrogen bonds and the chains are bound together forming aggregates which are insoluble in water. In very dilute solutions, the aggregated chains of amylose will precipitate; in more concentrated solutions, a gel will form. This essentially crystalline process of alignment, association and precipitation or gelling is known as retrogradation. Because of the linearity of amylose and its marked tendency to form associated aggregates, this material is insoluble in water and forms strong, flexible films.
In contrast, the highly branched chains of the amylopectin molecules cannot align and associate so readily. Consequently, amylopectin tends to be soluble in water, forming solutions that will not gel under normal conditions. However, prolonged aging or special conditions such as freezing may effect retrogradation in some dispersions containing amylopectin.
Principally due to these differences in the solubility properties of the two structural starch forms, previous attempts to produce water-insensitive starch fibers or films have been directed to starches containing substantial quantities of amylose. Thus, U.S. Pats. Nos. 2,902,336, 3,030,667, 3,336,429, and 3,116,351 among others, although differing in techniques for producing fibers, all have in common the use of starches containing at least 50%, and generally 80 to 100%, by weight amylose. The methods of these patents therefore rely on the linear chain amylose portion of the starch to provide the water-insensitive properties of the final fiber and any amylopectin present is treated as an impurity, tolerable in only minor quantities. However, it is well known in the art that such grades of starch containing 80 to 100% amylose do not occur naturally and are only obtained by subjecting starch to treatments wherein a substantial portion (i.e. that portion comprising amylopectin) is discarded, thereby rendering the manufacture and use of such fibers on a commercial scale economically disadvantageous.
Other methods for the preparation of starch containing fibers such as by plasticizing starch dispersions with softeners or fluxes to convert them into "pseudothermoplastics" or by thermally decomposing the starch to form a starch xanthate fiber have been taught in U.S. Pats. Nos. 2,570,449 and 3,497,584 respectively. Such methods require complicated processing conditions and some do not necessarily result in the production of water-insensitive fibers.
It is therefore an object of the present invention to provide a process for the production of water-insensitive starch fibers in which the presence of amylopectin is not deleterious.
It is another object to provide a process which produces starch fibers from starches which do not contain relatively high concentration of the linear chain polymer, amylose.
It is also an object to provide a process which produces starch fibers from naturally occurring starches, and hence is economical and efficient.
Another object is to provide such a process which produces starch fibers from 100% amylopectin.
Another object is to provide a process which produces starch fibers which are strong and durable as well as water-insensitive.
A further object is to provide a process whereby a variety of water-insoluble materials may be incorporated into a starch dispersion and subsequently encapsulated within the fiber matrix during its formation for the purpose of imparting a wide variety of functional characteristics to the final fiber.
Yet another object is to provide starch fibers which possess superior properties and which may be produced in discrete lengths and used as supplements to or replacements for natural cellulose fibers in a papermaking process.
These and other related objects will be apparent from the descriptions which follow.